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Dense-gas fractionation of mixtures of petroleum macromolecules
Fluid Phase Equilibria 224 (2004) 231–236
Dense-gas fractionation of mixtures of petroleum macromolecules William F. Edwards, Mark C. Thies∗ Department of Chemical Engineering, Clemson University, 127 Earle Hall, Clemson, SC 29634-0909, USA Received 5 April 2004; received in revised form 3 August 2004; accepted 4 August 2004
Abstract Dense-gas extraction (DGE) was used to fractionate a petroleum pitch feedstock of broad molecular weight distribution (MWD) into selected oligomeric constituents. The DGE process was carried out in a semibatch mode using the solvent toluene (Tc = 318.6 ◦ C, Pc = 41.1 bar) and a pitch charge of 10–15 g. A packed column with reflux produced the equivalent of several equilibrium stages. The apparatus was operated at near and supercritical temperatures, with the stillpot at 320 ◦ C and the column temperature increasing linearly up to the reflux condenser temperature of 360 ◦ C. Operating pressures were determined by the conditions required to extract a given oligomer and varied from 44 to 84 bar. Using a monomer-rich petroleum pitch feed (MWn = 299, PDI = 1.10), a dimer-rich extract with a purity approaching 90% (MWn = 521, PDI = 1.07) was obtained, with the remaining residue (MWn = 883, PDI = 1.24) consisting of 95% trimer and higher oligomers. MALDI-TOF mass spectrometry was used to determine the MWD of the feed pitch and pitch fractions, with a unique matrix being used to enhance the response of higher MW species. The combination of DGE for separation and MALDI for accurate MW information is a powerful new technique for the molecular characterization of insoluble, high MW fossil fuels. © 2004 Elsevier B.V. All rights reserved. Keywords: Petroleum; Pitch; Supercritical; Extraction; Vapor–liquid equilibria; Experimental method
1. Introduction Petroleum pitches are formed by the thermal polymerization of decant oil, a high molecular weight (MW), aromatic oil that is a byproduct of the catalytic cracking of petroleum distillates [1]. The polymerization reaction produces a material with a broad molecular weight distribution (MWD) ranging from approximately 200–2000. As shown in Fig. 1, these pitches are somewhat comparable to asphaltenes, but are more aromatic, contain fewer metallic and heteroatoms, and are only partially soluble in toluene [2,3]. When advanced carbon products are produced from pitches, their molecular composition acquires special importance. For example, when higher MW fractions are isolated from pitch, the large plate-like molecules can spontaneously align to form discotic liquid crystals (also called mesophase) ∗ Corresponding author. Tel.: +1 864 656 5424; fax: +1 864 656 0784. E-mail address: [email protected] (M.C. Thies).
0378-3812/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2004.08.004
that are the starting materials for high-thermal-conductivity carbon fibers [4]. On the other hand, other lower MW cuts are preferred as starting materials for the matrix phase of carboncarbon composites or for activated carbon fibers [5]. Unfortunately, the problem of determining fundamental molecular information for pitches is a difficult one that has yet to be solved. Thus, we can only relate pitch molecular composition to final carbon product properties in, at best, a qualitative sense. A significant barrier to an improved understanding of the molecular composition of pitches is the difficulty in separating pitches into narrow MW fractions that can serve as molecular calibration standards. Unfortunately, the classic technique for the separation of petroleum pitches, gel permeation chromatography [6,7], suffers from a number of disadvantages, including poor peak resolution and the incomplete solubility of higher MW pitches in even aggressive mobile phase solvents [7–9]. In an ongoing research project at Clemson, we are investigating the use of a multi-stage separation technique for the
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2. Experimental
Fig. 1. Representative molecular structures in petroleum pitch [2].
fractionation of heavy fossil fuels, particularly those that are only partially soluble in solvents and thus cannot be separated by more conventional means (e.g., petroleum pitch). We call the method dense-gas extraction (DGE). The basic concept of DGE was pioneered by Zosel for the fractionation of cod liver oil [10]; he called it destraction. The term gas extraction has also been used [11]. With DGE, the extractive solvent is a dense-gas in the vicinity of its critical temperature, with the pressures being high enough so that the solvent has significantly more extractive power than an ideal gas. Pressures can be either below those typically used in supercritical extraction or at supercritical conditions, depending on the solubility of the solute to be extracted into the solvent phase. Because the goal is to obtain narrow MW fractions, the dense-gas and solute are contacted in a packed column with a separation power equivalent to several equilibrium stages. Liquid reflux is generated by temperature changes either along or at the top of the column (phase behavior can be either conventional or retrograde) and serves to enhance product purity. A significant impediment to the fractionation of heavy fossil fuels has been their subsequent analysis and characterization. For example, Shi et al. [12] used DGE to fractionate petroleum residua with pentane, but because vapor pressure osmometry (VPO) was used for analysis of the cuts obtained, nothing other than average MW information was obtained for any of the fractions. Recently, we reported on our development of matrix-assisted, laser desorption/ionization (MALDI), time-of-flight (TOF) mass spectrometry (MS), or MALDI for short, for the MW determination of insoluble, higher MW pitches and other heavy fossil fuels [13]. To our knowledge, no other investigators have used the combination of DGE and MALDI for the fractionation and analysis/characterization of heavy fossil fuels.
A MALDI mass spectrum of the feed pitch used in this study is shown in Fig. 2. Note that the pitch is oligomeric in nature, with the monomer covering the range from 210 to 420, the dimer from 420 to 680, the trimer from 680 to 940, the tetramer from 940 to 1200, etc. (The trimer and tetramer are barely visible in Fig. 2 because of their low concentration.) A schematic of the DGE procedure used in this study is shown in Fig. 3. The feed pitch is charged to the DGE column, and the column is then operated at an initial pressure (i.e., Column Pressure 1) such that the monomer components are extracted in the overhead stream, but without extracting a significant amount of dimer. After several hours of operation at this initial pressure, the column pressure is increased to increase the solvent power of the dense-gas (i.e., Column Pressure 2) and thus extract the dimer species from the charge as overhead. At the end of the experiment, the unextracted, high MW portion of the feed charge remains as residue in the bottom of the column. Extraction of a trimer-rich cut is carried out in a similar manner, where the initial pressure (i.e., Column Pressure 3) is chosen such that monomer and dimer species are extracted in the overhead stream. The column pressure is subsequently increased (i.e., Column Pressure 4) to extract a trimer-rich cut as the overhead product. 2.1. Materials HPLC grade toluene (CAS 108-88-3) with a stated purity of 99.9% and HPLC grade 1,2,4-trichlorobenzene (CAS 120-82-1) with a stated purity of 99.6% was both obtained from Fisher Scientific. An isotropic petroleum pitch feedstock, produced by the thermal polymerization of FCC decant oil, was obtained from ConocoPhillips (see Fig. 2). This pitch is representative of a starting material that, after fractionation, can be used to manufacture carbon products such as fibers and composites. 2.2. Experimental apparatus and procedure A semibatch, multistage extraction unit was constructed to study the dense-gas extraction of pitches. The apparatus,
Fig. 2. MALDI mass spectrum of the petroleum pitch that served as the feedstock for DGE.
W.F. Edwards, M.C. Thies / Fluid Phase Equilibria 224 (2004) 231–236
233
Fig. 3. Flowsheet of the DGE process for the production of pitch oligomers.
which is shown in Fig. 4, is rated for 400 ◦ C and 200 bar and can hold a charge of up to 15 g of pitch. Briefly, the equipment consists of a pump and preheater for supplying solvent, a 1.8 cm i.d. × 115 cm tall column that contains a stillpot, a 70 cm high section of packing, and a reflux finger at the top. A regulating valve is used to control both the column pressure and the flow of overhead product exiting the column. For a typical experimental run, the stillpot (i.e., the first 23 cm of the column, which is packing-free) is charged with pitch. This is accomplished by loading the charge cartridge with pitch and introducing the cartridge into the bottom of the stillpot. The extraction solvent reservoir is filled with the desired solvent (in this work, toluene). A piston pump (Milton Roy Minipump, Model 92014803) delivers the solvent at a constant mass flow rate (typical flows are 1.5–10 g/min) and the desired column operating pressure. The compressed solvent then flows through the preheater, where it is heated to the desired dense-gas conditions. The dense-gas solvent flows
Fig. 4. Schematic of the dense-gas extraction apparatus.
into the bottom of the charge cartridge. Here a sintered disc distributes the entering solvent evenly across the pitch sample. The solvent mixes with the pitch charge in the stillpot, selectively extracting a portion of the pitch. The dense-gas mixture of solvent and extracted pitch then rises from the stillpot and enters the packed section of the column, which is filled with 4 mm random packing (Cannon Instrument Co., part no. 3947-A20). The temperatures of the stillpot and column are controlled with three PID controllers (Omega, Model CN77344) and a system of band heaters. Upon exiting the packed section, the dense-gas mixture contacts the reflux finger, which is housed in the top 14 cm of the column and serves as a condenser. If the temperature of the finger or the packed column is different from the stillpot temperature, a pitch-rich liquid phase can condense. This liquid phase flows down the column as reflux, further purifying the overhead vapor fraction. The overhead vapor product exits out the side of the column, adjacent to the top of the reflux finger, and is expanded to ambient pressures by means of the regulating valve (Autoclave Engineers, part no. 30VM4084GY). The product is then collected and condensed in a 500-ml glass kettle cooled in an ice bath. Typically, 5–10 samples are taken for 15–60 min each, with sample sizes ranging from 100 to 300 g each (and from 0.050 to 10 g each on a solvent-free basis), depending on the extraction conditions used. After an experiment is completed, the solvent flow to the column is stopped, the regulating valve is opened to allow the column to depressurize, and the column is allowed to cool. The charge cartridge is removed from the bottom of the stillpot. The residue taken from the cartridge is analyzed along with the overhead fractions. The construction of the DGE apparatus is described in greater detail elsewhere [14]. Column thermocouples were calibrated to an accuracy of ±1.0 ◦ C against a secondary standard platinum RTD (Burns Engineering, 200 Series), which is accurate to within ±0.1 ◦ C. The temperature of the column at a given location can be controlled to ±1.0 ◦ C, giving a total uncertainty in the reported temperatures of ±2 ◦ C. Column pressure was controlled by means of the regulating valve (see Fig. 4), which was actuated via a National Instruments NuDrive motion control device (Model 4CX-001) along with a DC servomotor
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and gear reducer (ECM Motor Co., Model 5471). The system pressure was measured with a pressure transducer (Heise, Model HPO) that was monitored by National Instruments Labview software. The transducer was calibrated against a Budenberg deadweight tester (Model 380H) to an accuracy of ±0.07 bar. The column pressure was maintained to within ±0.07 bar of the desired setpoint at all times, giving an uncertainty in the reported pressures of ±0.14 bar. 2.3. Analysis of Pitch Samples The compositions of the overhead fractions and residues collected from the DGE apparatus were analyzed using a Bruker Daltonics Autoflex MALDI-TOF mass spectrometer. Details of the procedures developed in our group for the analysis of insoluble, high MW pitches and fossil fuels by MALDI are given elsewhere [13]. The matrix 7,7,8,8tetracyanoquinodimethane (TCNQ) was used to enhance the response of higher MW species. For analysis of the feed pitch and the residue, which are not completely soluble in solvents, the sample and matrix were combined and ground in a mortar and pestle; the resultant powder was then deposited on the target cell as a film on the surface of a bead of water. Most of the water was withdrawn from under the film with a pipette, and the film was allowed to completely dry before MALDI analysis. In the case of the lower MW overhead fractions collected from the DGE apparatus, a film of pure TCNQ was cast onto the target cell using the method described above, and a solution of the pitch fraction dissolved in carbon disulfide was applied directly to the film and allowed to dry before analysis. For samples that were completely soluble in carbon disulfide, the “solution method” was preferred to the “powder method” because it consumed far less sample. For each MALDI analysis, the applied laser power was optimized by trial and error. Too high a laser power can cause poor resolution, detector saturation, and species fragmentation. The spectra obtained are the summation of 100 laser shots and acquisitions. Although MALDI can be used to accurately determine the MWs of the species present in pitches, the technique has yet to be established as a quantitative method of analysis. In other words, the relationships between the intensity, or height, of a
MALDI peak and the mass or mole fraction of that species in the pitch mixture is unknown. In this study, we have chosen to report results in terms of area percent. To obtain the area fractions by MALDI, the MS data is integrated by dividing the data into MW ranges, with each range representing an oligomer. Monomer has been defined as the MW range from 210 to 420, dimer as 420 to 680, trimer as 680 to 940, and tetramer as 940 to 1200. In addition to determining the relative peak areas, the number-average MW (MWn ) and the polydispersity index (PDI) were also calculated.
3. Results and discussion Preliminary experiments showed that a monomer fraction with 99+ % purity was easily obtainable using DGE. Because conventional separation methods can be used to recover species in the monomer MW range, the focus of our experiments was on the recovery of relatively pure dimer and trimer fractions. For all experiments, the charge cartridge was filled to capacity with 10 to 15 g of the feed pitch, inserted into the apparatus, and the column was purged with nitrogen for at least 12 h. The flow rate of the solvent toluene was 3.9 g/min, and the preheater outlet temperature was controlled to 320 ◦ C. The stillpot was maintained at 320 ◦ C, the reflux finger at 360 ◦ C, and a linear temperature gradient was established between these two temperatures across the column packing. The above temperatures and column temperature profile were kept constant during the experiments, and the column pressure was manipulated to vary the solvent power of the dense-gas. Column pressures were chosen empirically by using the results from preliminary experiments. Overhead products, or extracts, were collected at the desired pressures for 30–60 min. After the conclusion of the experiment, the charge cartridge was removed and the remaining pitch was defined as the residue. Our efforts to recover a dimer-rich fraction from the whole feed pitch are summarized in Table 1, where results are shown for a typical experiment. As shown in the table and as was discussed earlier, the purpose of the first three extract collections at 44.1 bar (Overheads 1–3) was to remove most of the monomer species, albeit without also stripping off an
Table 1 Isolation of a dimer-rich cut of petroleum pitch by DGE Fraction
chargea
Pitch Overhead 1 Overhead 2 Overhead 3 Overhead 4 Overhead 5 Overhead 6 Overhead 7 Residue a
Pressure (bar)
– 44.1 44.1 44.1 52.7 52.7 52.7 52.7 –
Toluene flow rate was 3.9 g/min.
Mass (g)
10.094 4.111 2.546 0.411 0.264 0.150 0.161 0.089 0.069
Collection time (min)
– 60 60 60 30 30 30 30 –
Area percent by MALDI-MS Monomer
Dimer
Trimer
Tetramer+
87.9 79.9 63.8 43.4 11.2 9.1 5.8 5.7 2.1
10.1 16.3 24.8 36.9 70.3 83.8 86.1 87.2 3.8
1.6 3.2 8.7 12.4 8.5 2.4 2.3 2.2 31.7
0.4 0.7 2.7 7.3 10.0 4.6 5.8 4.9 62.4
W.F. Edwards, M.C. Thies / Fluid Phase Equilibria 224 (2004) 231–236
Fig. 5. MALDI mass spectrum of Overhead 7 extract (Table 1) obtained by DGE, using a 320–360 ◦ C temperature gradient and a pressure of 52.7 bar. The dimer is present in this cut at a concentration of 87.2 area %.
Fig. 6. MALDI mass spectrum of the stillpot residue (see Table 1) obtained by DGE, using a 320–360 ◦ C temperature gradient and a pressure of 52.7 bar. The residue consists of 95% trimer and higher oligomers.
excessive amount of dimer and higher oligomers. For the collection of dimer-rich fractions, the pressure was then increased to 52.7 bar. The experiment was continued for two additional hours, with the collected extract being accumulated in four 30 min allotments (Overheads 4–7). A representative MALDI spectrum of a dimer-rich extract (i.e., for Overhead 7) is shown in Fig. 5, while the mass spectrum for the residue remaining in the stillpot after a run is shown as Fig. 6. Thus, we see that DGE can be used to produce a dimer-rich cut with a purity approaching 90%. Furthermore,
235
Fig. 7. Isolation of a dimer-rich fraction from the petroleum pitch shown in Fig. 2 by DGE (black) vs. prep-scale chromatography (grey).
the remaining residue has been stripped of most of the lower MW pitch components and thus consists of ∼95% trimer and higher oligomers. In Fig. 7, the MWD of a dimer cut obtained in dg-sized quantities by DGE is compared to a fraction that was produced in mg-sized quantities by prep-scale, silica gel chromatography [15]. Clearly, DGE is preferred in terms of both purity and yield. The results of our initial efforts to obtain a trimer-rich fraction of pitch are summarized in Table 2, where results are shown for a typical experiment. The DGE process was initially carried out for 3 at a pressure of 59.6 bar, with the objective being to strip the monomer and dimer from the pitch charge. The pressure was then raised to 83.7 bar in order to collect the trimer-rich fractions. Unfortunately, not enough of the dimer species was removed prior to raising the pressure; thus, the area percentage of trimer obtained did not exceed 40.4%. Nevertheless, the significant increase in trimer concentration obtained between the feed and the Overhead 7 cut (i.e., from 1.6 to 40.4 area %) indicates the potential of DGE for isolating higher MW oligomers from petroleum pitches. It should also be noted that prep-scale chromatography was unsuccessful in the recovery of any trimer-enriched fractions [15]. Additional research would be required to determine whether the existing setup is capable of producing higher trimer purities, or whether a new DGE column with a higher packing height and/or continuous-flow capabilities would be required.
Table 2 Isolation of a trimer-rich cut of petroleum pitch by DGE Fraction
Pressure (bar)
Mass (g)
Collection time (min)
Area percent by MALDI-MS Monomer
Dimer
Trimer
Tetramer+
Pitch Chargea Overhead 1 Overhead 2 Overhead 3 Overhead 4 Overhead 5 Overhead 6 Overhead 7 Residue
– 59.6 59.6 59.6 83.7 83.7 83.7 83.7 –
12.866 11.299 0.305 0.227 0.349 0.139 0.087 0.068 0.071
– 60 60 60 30 30 30 30 –
87.9 18.4 1.9 7.6 7.8 9.3 17.2 11.9 4.9
10.1 30.5 60.3 84.8 62.9 54.9 43.9 43.5 25.5
1.6 21.7 17.3 5.9 25.9 32.3 35.1 40.4 22.8
0.4 29.4 20.4 1.7 3.3 3.6 3.8 4.1 46.8
a
Toluene flow rate was 3.9/min.
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4. Conclusions
References
DGE was evaluated for the fractionation of high MW, carbonaceous pitches and other heavy fossils that cannot be separated by chromatographic techniques such as GPC and liquid chromatography. Beginning with a petroleum pitch of broad MWD, we have isolated a dimer-rich fraction (MWn = 521, PDI = 1.07) of about 90% purity and recovered a high MW residue (MW = 883, PDI = 1.24) consisting almost exclusively of trimer and higher oligomers. The combination of DGE for separation and MALDI for accurate MW information is a powerful new technique for the molecular characterization of insoluble, high MW fossil fuels.
[1] D.D. Edie, in: J.L. Figueiredo (Ed.), Carbon Fibers Filaments and Composites, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1990, pp. 1–30. [2] E.M. Dickinson, Fuel 64 (1985) 704–706. [3] M.S. Zhuang, M.C. Thies, Energy Fuels 14 (2000) 70–75. [4] D.D. Edie, Carbon 36 (1998) 345–362. [5] Y.V. Basova, D.D. Edie, Y.S. Lee, L.K. Reid, S.K. Ryu, Carbon 42 (2004) 485–495. [6] I.C. Lewis, B.A. Petro, J. Polym. Sci. Polym. Chem. Ed. 14 (1976) 1975–1985. [7] R.A. Greinke, Carbon 24 (1986) 677–686. [8] F.M. Dauch´e, G. Bola˜nos, A. Blasig, M.C. Thies, Carbon 36 (1998) 953–961. [9] M.S. Zhuang, K. Gast, M.C. Thies, Ind. Eng. Chem. Res. 41 (2002) 78–84. [10] K. Zosel, in: G.M., Schneider, E., Stahl, G., Wilke, (Eds.), Extraction with Supercritical Gases, Verlag Chemie, Weinheim, 1980, pp. 1–23. [11] G. Brunner, Gas Extraction: An Introduction to Fundamentals of Supercritical Fluids and the Application to Separation Processes, Springer-Verlag, New York, 1994. [12] T.P. Shi, Y.X. Hu, Z.M. Xu, T. Su, R.A. Wang, Ind. Eng. Chem. Res. 36 (1997) 3988–3992. [13] W.F. Edwards, L. Jin, M.C. Thies, Carbon 41 (2003) 2761–2768. [14] W.F. Edwards, M.C. Thies, Energy Fuel. Submitted for publication. [15] W.M. Southard, ConocoPhillips, Ponca City, OK, private communication, 2002.
Acknowledgments This work was supported by ConocoPhillips Inc., the Engineering Research Centers Program of the National Science Foundation under NSF Award Number EEC-9731680, and by ERC Corp. The authors thank Liwen Jin, Jaquelin Moseley, and Robert Hammett for their assistance with the MALDI analyses.
Dense-gas fractionation of mixtures of petroleum macromolecules William F. Edwards, Mark C. Thies∗ Department of Chemical Engineering, Clemson University, 127 Earle Hall, Clemson, SC 29634-0909, USA Received 5 April 2004; received in revised form 3 August 2004; accepted 4 August 2004
Abstract Dense-gas extraction (DGE) was used to fractionate a petroleum pitch feedstock of broad molecular weight distribution (MWD) into selected oligomeric constituents. The DGE process was carried out in a semibatch mode using the solvent toluene (Tc = 318.6 ◦ C, Pc = 41.1 bar) and a pitch charge of 10–15 g. A packed column with reflux produced the equivalent of several equilibrium stages. The apparatus was operated at near and supercritical temperatures, with the stillpot at 320 ◦ C and the column temperature increasing linearly up to the reflux condenser temperature of 360 ◦ C. Operating pressures were determined by the conditions required to extract a given oligomer and varied from 44 to 84 bar. Using a monomer-rich petroleum pitch feed (MWn = 299, PDI = 1.10), a dimer-rich extract with a purity approaching 90% (MWn = 521, PDI = 1.07) was obtained, with the remaining residue (MWn = 883, PDI = 1.24) consisting of 95% trimer and higher oligomers. MALDI-TOF mass spectrometry was used to determine the MWD of the feed pitch and pitch fractions, with a unique matrix being used to enhance the response of higher MW species. The combination of DGE for separation and MALDI for accurate MW information is a powerful new technique for the molecular characterization of insoluble, high MW fossil fuels. © 2004 Elsevier B.V. All rights reserved. Keywords: Petroleum; Pitch; Supercritical; Extraction; Vapor–liquid equilibria; Experimental method
1. Introduction Petroleum pitches are formed by the thermal polymerization of decant oil, a high molecular weight (MW), aromatic oil that is a byproduct of the catalytic cracking of petroleum distillates [1]. The polymerization reaction produces a material with a broad molecular weight distribution (MWD) ranging from approximately 200–2000. As shown in Fig. 1, these pitches are somewhat comparable to asphaltenes, but are more aromatic, contain fewer metallic and heteroatoms, and are only partially soluble in toluene [2,3]. When advanced carbon products are produced from pitches, their molecular composition acquires special importance. For example, when higher MW fractions are isolated from pitch, the large plate-like molecules can spontaneously align to form discotic liquid crystals (also called mesophase) ∗ Corresponding author. Tel.: +1 864 656 5424; fax: +1 864 656 0784. E-mail address: [email protected] (M.C. Thies).
0378-3812/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2004.08.004
that are the starting materials for high-thermal-conductivity carbon fibers [4]. On the other hand, other lower MW cuts are preferred as starting materials for the matrix phase of carboncarbon composites or for activated carbon fibers [5]. Unfortunately, the problem of determining fundamental molecular information for pitches is a difficult one that has yet to be solved. Thus, we can only relate pitch molecular composition to final carbon product properties in, at best, a qualitative sense. A significant barrier to an improved understanding of the molecular composition of pitches is the difficulty in separating pitches into narrow MW fractions that can serve as molecular calibration standards. Unfortunately, the classic technique for the separation of petroleum pitches, gel permeation chromatography [6,7], suffers from a number of disadvantages, including poor peak resolution and the incomplete solubility of higher MW pitches in even aggressive mobile phase solvents [7–9]. In an ongoing research project at Clemson, we are investigating the use of a multi-stage separation technique for the
232
W.F. Edwards, M.C. Thies / Fluid Phase Equilibria 224 (2004) 231–236
2. Experimental
Fig. 1. Representative molecular structures in petroleum pitch [2].
fractionation of heavy fossil fuels, particularly those that are only partially soluble in solvents and thus cannot be separated by more conventional means (e.g., petroleum pitch). We call the method dense-gas extraction (DGE). The basic concept of DGE was pioneered by Zosel for the fractionation of cod liver oil [10]; he called it destraction. The term gas extraction has also been used [11]. With DGE, the extractive solvent is a dense-gas in the vicinity of its critical temperature, with the pressures being high enough so that the solvent has significantly more extractive power than an ideal gas. Pressures can be either below those typically used in supercritical extraction or at supercritical conditions, depending on the solubility of the solute to be extracted into the solvent phase. Because the goal is to obtain narrow MW fractions, the dense-gas and solute are contacted in a packed column with a separation power equivalent to several equilibrium stages. Liquid reflux is generated by temperature changes either along or at the top of the column (phase behavior can be either conventional or retrograde) and serves to enhance product purity. A significant impediment to the fractionation of heavy fossil fuels has been their subsequent analysis and characterization. For example, Shi et al. [12] used DGE to fractionate petroleum residua with pentane, but because vapor pressure osmometry (VPO) was used for analysis of the cuts obtained, nothing other than average MW information was obtained for any of the fractions. Recently, we reported on our development of matrix-assisted, laser desorption/ionization (MALDI), time-of-flight (TOF) mass spectrometry (MS), or MALDI for short, for the MW determination of insoluble, higher MW pitches and other heavy fossil fuels [13]. To our knowledge, no other investigators have used the combination of DGE and MALDI for the fractionation and analysis/characterization of heavy fossil fuels.
A MALDI mass spectrum of the feed pitch used in this study is shown in Fig. 2. Note that the pitch is oligomeric in nature, with the monomer covering the range from 210 to 420, the dimer from 420 to 680, the trimer from 680 to 940, the tetramer from 940 to 1200, etc. (The trimer and tetramer are barely visible in Fig. 2 because of their low concentration.) A schematic of the DGE procedure used in this study is shown in Fig. 3. The feed pitch is charged to the DGE column, and the column is then operated at an initial pressure (i.e., Column Pressure 1) such that the monomer components are extracted in the overhead stream, but without extracting a significant amount of dimer. After several hours of operation at this initial pressure, the column pressure is increased to increase the solvent power of the dense-gas (i.e., Column Pressure 2) and thus extract the dimer species from the charge as overhead. At the end of the experiment, the unextracted, high MW portion of the feed charge remains as residue in the bottom of the column. Extraction of a trimer-rich cut is carried out in a similar manner, where the initial pressure (i.e., Column Pressure 3) is chosen such that monomer and dimer species are extracted in the overhead stream. The column pressure is subsequently increased (i.e., Column Pressure 4) to extract a trimer-rich cut as the overhead product. 2.1. Materials HPLC grade toluene (CAS 108-88-3) with a stated purity of 99.9% and HPLC grade 1,2,4-trichlorobenzene (CAS 120-82-1) with a stated purity of 99.6% was both obtained from Fisher Scientific. An isotropic petroleum pitch feedstock, produced by the thermal polymerization of FCC decant oil, was obtained from ConocoPhillips (see Fig. 2). This pitch is representative of a starting material that, after fractionation, can be used to manufacture carbon products such as fibers and composites. 2.2. Experimental apparatus and procedure A semibatch, multistage extraction unit was constructed to study the dense-gas extraction of pitches. The apparatus,
Fig. 2. MALDI mass spectrum of the petroleum pitch that served as the feedstock for DGE.
W.F. Edwards, M.C. Thies / Fluid Phase Equilibria 224 (2004) 231–236
233
Fig. 3. Flowsheet of the DGE process for the production of pitch oligomers.
which is shown in Fig. 4, is rated for 400 ◦ C and 200 bar and can hold a charge of up to 15 g of pitch. Briefly, the equipment consists of a pump and preheater for supplying solvent, a 1.8 cm i.d. × 115 cm tall column that contains a stillpot, a 70 cm high section of packing, and a reflux finger at the top. A regulating valve is used to control both the column pressure and the flow of overhead product exiting the column. For a typical experimental run, the stillpot (i.e., the first 23 cm of the column, which is packing-free) is charged with pitch. This is accomplished by loading the charge cartridge with pitch and introducing the cartridge into the bottom of the stillpot. The extraction solvent reservoir is filled with the desired solvent (in this work, toluene). A piston pump (Milton Roy Minipump, Model 92014803) delivers the solvent at a constant mass flow rate (typical flows are 1.5–10 g/min) and the desired column operating pressure. The compressed solvent then flows through the preheater, where it is heated to the desired dense-gas conditions. The dense-gas solvent flows
Fig. 4. Schematic of the dense-gas extraction apparatus.
into the bottom of the charge cartridge. Here a sintered disc distributes the entering solvent evenly across the pitch sample. The solvent mixes with the pitch charge in the stillpot, selectively extracting a portion of the pitch. The dense-gas mixture of solvent and extracted pitch then rises from the stillpot and enters the packed section of the column, which is filled with 4 mm random packing (Cannon Instrument Co., part no. 3947-A20). The temperatures of the stillpot and column are controlled with three PID controllers (Omega, Model CN77344) and a system of band heaters. Upon exiting the packed section, the dense-gas mixture contacts the reflux finger, which is housed in the top 14 cm of the column and serves as a condenser. If the temperature of the finger or the packed column is different from the stillpot temperature, a pitch-rich liquid phase can condense. This liquid phase flows down the column as reflux, further purifying the overhead vapor fraction. The overhead vapor product exits out the side of the column, adjacent to the top of the reflux finger, and is expanded to ambient pressures by means of the regulating valve (Autoclave Engineers, part no. 30VM4084GY). The product is then collected and condensed in a 500-ml glass kettle cooled in an ice bath. Typically, 5–10 samples are taken for 15–60 min each, with sample sizes ranging from 100 to 300 g each (and from 0.050 to 10 g each on a solvent-free basis), depending on the extraction conditions used. After an experiment is completed, the solvent flow to the column is stopped, the regulating valve is opened to allow the column to depressurize, and the column is allowed to cool. The charge cartridge is removed from the bottom of the stillpot. The residue taken from the cartridge is analyzed along with the overhead fractions. The construction of the DGE apparatus is described in greater detail elsewhere [14]. Column thermocouples were calibrated to an accuracy of ±1.0 ◦ C against a secondary standard platinum RTD (Burns Engineering, 200 Series), which is accurate to within ±0.1 ◦ C. The temperature of the column at a given location can be controlled to ±1.0 ◦ C, giving a total uncertainty in the reported temperatures of ±2 ◦ C. Column pressure was controlled by means of the regulating valve (see Fig. 4), which was actuated via a National Instruments NuDrive motion control device (Model 4CX-001) along with a DC servomotor
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and gear reducer (ECM Motor Co., Model 5471). The system pressure was measured with a pressure transducer (Heise, Model HPO) that was monitored by National Instruments Labview software. The transducer was calibrated against a Budenberg deadweight tester (Model 380H) to an accuracy of ±0.07 bar. The column pressure was maintained to within ±0.07 bar of the desired setpoint at all times, giving an uncertainty in the reported pressures of ±0.14 bar. 2.3. Analysis of Pitch Samples The compositions of the overhead fractions and residues collected from the DGE apparatus were analyzed using a Bruker Daltonics Autoflex MALDI-TOF mass spectrometer. Details of the procedures developed in our group for the analysis of insoluble, high MW pitches and fossil fuels by MALDI are given elsewhere [13]. The matrix 7,7,8,8tetracyanoquinodimethane (TCNQ) was used to enhance the response of higher MW species. For analysis of the feed pitch and the residue, which are not completely soluble in solvents, the sample and matrix were combined and ground in a mortar and pestle; the resultant powder was then deposited on the target cell as a film on the surface of a bead of water. Most of the water was withdrawn from under the film with a pipette, and the film was allowed to completely dry before MALDI analysis. In the case of the lower MW overhead fractions collected from the DGE apparatus, a film of pure TCNQ was cast onto the target cell using the method described above, and a solution of the pitch fraction dissolved in carbon disulfide was applied directly to the film and allowed to dry before analysis. For samples that were completely soluble in carbon disulfide, the “solution method” was preferred to the “powder method” because it consumed far less sample. For each MALDI analysis, the applied laser power was optimized by trial and error. Too high a laser power can cause poor resolution, detector saturation, and species fragmentation. The spectra obtained are the summation of 100 laser shots and acquisitions. Although MALDI can be used to accurately determine the MWs of the species present in pitches, the technique has yet to be established as a quantitative method of analysis. In other words, the relationships between the intensity, or height, of a
MALDI peak and the mass or mole fraction of that species in the pitch mixture is unknown. In this study, we have chosen to report results in terms of area percent. To obtain the area fractions by MALDI, the MS data is integrated by dividing the data into MW ranges, with each range representing an oligomer. Monomer has been defined as the MW range from 210 to 420, dimer as 420 to 680, trimer as 680 to 940, and tetramer as 940 to 1200. In addition to determining the relative peak areas, the number-average MW (MWn ) and the polydispersity index (PDI) were also calculated.
3. Results and discussion Preliminary experiments showed that a monomer fraction with 99+ % purity was easily obtainable using DGE. Because conventional separation methods can be used to recover species in the monomer MW range, the focus of our experiments was on the recovery of relatively pure dimer and trimer fractions. For all experiments, the charge cartridge was filled to capacity with 10 to 15 g of the feed pitch, inserted into the apparatus, and the column was purged with nitrogen for at least 12 h. The flow rate of the solvent toluene was 3.9 g/min, and the preheater outlet temperature was controlled to 320 ◦ C. The stillpot was maintained at 320 ◦ C, the reflux finger at 360 ◦ C, and a linear temperature gradient was established between these two temperatures across the column packing. The above temperatures and column temperature profile were kept constant during the experiments, and the column pressure was manipulated to vary the solvent power of the dense-gas. Column pressures were chosen empirically by using the results from preliminary experiments. Overhead products, or extracts, were collected at the desired pressures for 30–60 min. After the conclusion of the experiment, the charge cartridge was removed and the remaining pitch was defined as the residue. Our efforts to recover a dimer-rich fraction from the whole feed pitch are summarized in Table 1, where results are shown for a typical experiment. As shown in the table and as was discussed earlier, the purpose of the first three extract collections at 44.1 bar (Overheads 1–3) was to remove most of the monomer species, albeit without also stripping off an
Table 1 Isolation of a dimer-rich cut of petroleum pitch by DGE Fraction
chargea
Pitch Overhead 1 Overhead 2 Overhead 3 Overhead 4 Overhead 5 Overhead 6 Overhead 7 Residue a
Pressure (bar)
– 44.1 44.1 44.1 52.7 52.7 52.7 52.7 –
Toluene flow rate was 3.9 g/min.
Mass (g)
10.094 4.111 2.546 0.411 0.264 0.150 0.161 0.089 0.069
Collection time (min)
– 60 60 60 30 30 30 30 –
Area percent by MALDI-MS Monomer
Dimer
Trimer
Tetramer+
87.9 79.9 63.8 43.4 11.2 9.1 5.8 5.7 2.1
10.1 16.3 24.8 36.9 70.3 83.8 86.1 87.2 3.8
1.6 3.2 8.7 12.4 8.5 2.4 2.3 2.2 31.7
0.4 0.7 2.7 7.3 10.0 4.6 5.8 4.9 62.4
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Fig. 5. MALDI mass spectrum of Overhead 7 extract (Table 1) obtained by DGE, using a 320–360 ◦ C temperature gradient and a pressure of 52.7 bar. The dimer is present in this cut at a concentration of 87.2 area %.
Fig. 6. MALDI mass spectrum of the stillpot residue (see Table 1) obtained by DGE, using a 320–360 ◦ C temperature gradient and a pressure of 52.7 bar. The residue consists of 95% trimer and higher oligomers.
excessive amount of dimer and higher oligomers. For the collection of dimer-rich fractions, the pressure was then increased to 52.7 bar. The experiment was continued for two additional hours, with the collected extract being accumulated in four 30 min allotments (Overheads 4–7). A representative MALDI spectrum of a dimer-rich extract (i.e., for Overhead 7) is shown in Fig. 5, while the mass spectrum for the residue remaining in the stillpot after a run is shown as Fig. 6. Thus, we see that DGE can be used to produce a dimer-rich cut with a purity approaching 90%. Furthermore,
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Fig. 7. Isolation of a dimer-rich fraction from the petroleum pitch shown in Fig. 2 by DGE (black) vs. prep-scale chromatography (grey).
the remaining residue has been stripped of most of the lower MW pitch components and thus consists of ∼95% trimer and higher oligomers. In Fig. 7, the MWD of a dimer cut obtained in dg-sized quantities by DGE is compared to a fraction that was produced in mg-sized quantities by prep-scale, silica gel chromatography [15]. Clearly, DGE is preferred in terms of both purity and yield. The results of our initial efforts to obtain a trimer-rich fraction of pitch are summarized in Table 2, where results are shown for a typical experiment. The DGE process was initially carried out for 3 at a pressure of 59.6 bar, with the objective being to strip the monomer and dimer from the pitch charge. The pressure was then raised to 83.7 bar in order to collect the trimer-rich fractions. Unfortunately, not enough of the dimer species was removed prior to raising the pressure; thus, the area percentage of trimer obtained did not exceed 40.4%. Nevertheless, the significant increase in trimer concentration obtained between the feed and the Overhead 7 cut (i.e., from 1.6 to 40.4 area %) indicates the potential of DGE for isolating higher MW oligomers from petroleum pitches. It should also be noted that prep-scale chromatography was unsuccessful in the recovery of any trimer-enriched fractions [15]. Additional research would be required to determine whether the existing setup is capable of producing higher trimer purities, or whether a new DGE column with a higher packing height and/or continuous-flow capabilities would be required.
Table 2 Isolation of a trimer-rich cut of petroleum pitch by DGE Fraction
Pressure (bar)
Mass (g)
Collection time (min)
Area percent by MALDI-MS Monomer
Dimer
Trimer
Tetramer+
Pitch Chargea Overhead 1 Overhead 2 Overhead 3 Overhead 4 Overhead 5 Overhead 6 Overhead 7 Residue
– 59.6 59.6 59.6 83.7 83.7 83.7 83.7 –
12.866 11.299 0.305 0.227 0.349 0.139 0.087 0.068 0.071
– 60 60 60 30 30 30 30 –
87.9 18.4 1.9 7.6 7.8 9.3 17.2 11.9 4.9
10.1 30.5 60.3 84.8 62.9 54.9 43.9 43.5 25.5
1.6 21.7 17.3 5.9 25.9 32.3 35.1 40.4 22.8
0.4 29.4 20.4 1.7 3.3 3.6 3.8 4.1 46.8
a
Toluene flow rate was 3.9/min.
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4. Conclusions
References
DGE was evaluated for the fractionation of high MW, carbonaceous pitches and other heavy fossils that cannot be separated by chromatographic techniques such as GPC and liquid chromatography. Beginning with a petroleum pitch of broad MWD, we have isolated a dimer-rich fraction (MWn = 521, PDI = 1.07) of about 90% purity and recovered a high MW residue (MW = 883, PDI = 1.24) consisting almost exclusively of trimer and higher oligomers. The combination of DGE for separation and MALDI for accurate MW information is a powerful new technique for the molecular characterization of insoluble, high MW fossil fuels.
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Acknowledgments This work was supported by ConocoPhillips Inc., the Engineering Research Centers Program of the National Science Foundation under NSF Award Number EEC-9731680, and by ERC Corp. The authors thank Liwen Jin, Jaquelin Moseley, and Robert Hammett for their assistance with the MALDI analyses.